Interpretive Summary: Scientists have a wide array of computer models for predicting the fate and transport of agrochemicals applied to soil. Before these models can be accurately applied to specific situations, the parameters required for the model need to be measured or estimated for the soil of interest. One group of models, that is very useful for estimating rapid movement of water and chemicals through undisturbed natural soil, has been little used outside of the laboratory because of the difficulty in measuring the required model parameters. This research extended a method we developed previously to include the estimate of a third model parameter. We show that the new method gives model parameter estimates that are as reliable as our earlier approach. Now that the new method has been validated, scientists will be able to use the method with confidence in estimating parameter values for a wide range of soils and locations. The method will allow more accurate modeling of water and chemical transport through soils and better estimates of potential ground water contamination by agrochemicals.

Technical Abstract:
Rapid and nondestructive methods for determining solute transport properties are useful in many soil science applications. Recently, a series of field methods and a time domain reflectometry (TDR) method that could estimate some of the mobile/immobile model (MIM) parameters, immobile water content WIM and mass exchange coefficient alpha, have been reported. The first objective of this study was to determine an additional parameter, dispersion coefficient (O), using the TDR method. The three MIM parameters were estimated from the TDR measured data, and the estimated parameters were compared with the estimated parameters from the effluent data. The second objective was to determine whether the TDR determined parameters from the surface 2-cm soil layer could be used to predict effluent breakthrough curves (BTC) at the 20-cm depth. The TDR determined parameters were used to calculate effluent BTCs using the CXTFIT computer program. Parameters obtained by curve fitting of the three parameters simultaneously using TDR data were not similar to the parameters obtained form the effluent BTCs. The parameter estimations were improved by fixing one or two independently determined parameter(s) before curve fitting for the remaining unknown parameter(s). The calculated BTCs were similar to the observed BTCs with coefficient of determination r**2 being 0.99 and root mean square error (RMSE) being 0.036. The TDR data obtained from shallow soil layers were successfully used to describe solute transport through undisturbed soil cores.